Side-illumination type optical fiber

ABSTRACT

A side light type optical fiber, includes a core and a cladding disposed around the core, the cladding including a transparent first layer contacting the core, and a light diffusive second layer formed around the first layer, the layers being integrally molded.

TECHNICAL FIELD

The present invention relates to a side light type optical fiber.Particularly, it relates to a side light type optical fiber, which emitslight introduced from at least one end of a core in a longitudinaldirection through a cladding surrounding the core.

BACKGROUND ART

As is already known to the art, a discharge tube, like a fluorescenttube, emits visible light having a specified wave length region, and isused for illumination purposes. When the discharge tube is a neon tube,the tube is often employed for advertisement or ornamental use in theform of neon signs. The discharge tube emits light when applying anelectric discharge. It also emits heat together with light. The heat andleakage of electricity should be considered when using the dischargetube. The discharge tube, as the result, can not be used forilluminating or displaying in water.

In order to realize the illumination or display in water, anilluminating device in which its light source is placed separate from aplace to be illuminated or displayed has been recently suggested. Theilluminating device comprises a light source which is separately placedfrom an illuminating or displaying place, and an optical fiber forillumination or display, placed near or at the illuminating ordisplaying place. The optical fiber generally includes a core at acenter portion, in which a light introduced from one end of the fiber istransmitted to the other end, and a cladding having lower refractiveindex than the core, disposed around the core.

Among the optical fibers, there has been known a side light type opticalfiber which can emit light from its side portion. The side light typeoptical fiber is explained by reference with FIG. 4. The optical fiber20 is flexible and includes a core 21 formed from acrylic resin or thelike, and a cladding 22 formed from Polytetrafluoroethylene availablecommercially under the name Teflon™ from E.I. Dupont de Nemours andCompany and the like, as disclosed in U.S. Pat. No. 4,422,719. Thecladding 22 uniformly contains light diffusive particles, such as metaloxide particles (e.g. titanium dioxide particles) in an amount of 2 to10% by weight. In addition, Japanese Kokai Publication Hei-10(1998)-148725 discloses an optical fiber which is obtained byco-extruding a melted fluoropolymer containing 50 to 4,000 ppm of atleast one light diffusive additive with a crosslinkable resin mixturefor core. WO 98/08024 also discloses an optical fiber which is formed bymelt-casting a semi-transparent cladding material containing white oranother color pigment on a surface of a core. The optical fibersmentioned above can emit a light through the cladding, when light isintroduced from one or both ends of the fiber to transmit within thefiber.

It is also known that the cladding layer contains another layer lightdiffusive layer. For example, Japanese Kokai 2000-131530 discloses thata cladding layer is divided into two layers, one of which contains lightdiffusive particles to constitute a light diffusive layer and the otheris a transparent layer not containing light diffusive particles, formedon the light diffusive layer. The two layers are integrally formed byco-extrusion. In this technique, the light diffusive layer is directlycontacted with the core.

In the construction obtained in Japanese Kokai 2000-131530, a lateralluminance of the fiber is effectively enhanced in especially a portionnear the light source, but the luminance would attenuate as parting fromthe light source. This is because the greater the distance from thelight source, the more the luminance attenuates. Accordingly, theoptical fiber disclosed in Japanese Kokai 2000-131530 is not effectivelyused for an illumination device having a long fiber length of 10 m ormore, when the fiber is used as a light illuminant.

SUMMARY OF INVENTION

The present invention, as attaining the above-mentioned object, providesa side light type optical fiber (referred sometimes to as merely“optical fiber”), which comprises a core and a cladding disposed aroundthe core, the cladding is including a transparent first layer contactingthe core, and a light diffusive second layer formed around the firstlayer, the both layers being integrally molded. In the presentinvention, the first layer preferably has a thickness of 50 to 300 μm.The core preferably has a diameter of 5 to 30 mm. It is also preferredthat the cladding has a dual layer structure formed by a co-extrusionmethod of two materials for the first and second layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A schematic cross sectional view of the side light type opticalfiber of the present invention.

FIG. 2 A graph showing a result of test of side light luminance ofExamples and Comparative Examples. This figure shows a change ofluminance against distance from a light source.

FIG. 3 A graph showing a result of test of side light luminance ofExamples and Comparative Examples. This figure shows a change ofluminance against a measuring angle at 2 mm away from a light source.

FIG. 4 A schematic cross sectional view of the side light type opticalfiber of a prior art.

DETAILED DESCRIPTION

The long side light type optical fiber with 10 m or more length, canalso be obtained by covering an optical fiber having a transparentsingle-layer cladding on the core with a light diffusivesemi-transparent resin layer so as to enhance uniformity of luminanceover a longitudinal direction and to emit light brightly. This isbecause an optical fiber including a core and a transparent single-layercladding covering on the core can transmit light from one end to theother end in a longitudinal direction without leaking light from asurface of the cladding, that is, side face. This is because lightintroduced in the fiber is effectively transmitted by total internalreflection at an interface between the cladding having a relativelylower refractive index and the core having a relatively higherrefractive index.

On the other hand, in case where the core has a relatively largerdiameter, for example more than 3 mm, the light in the fiber leaksslightly out from the side face to illuminate a little over thelongitudinal dimension, even if the cladding does not contain lightdiffusive particles. The larger the diameter of the core, the more thephenomenon occurs, because more light which does not meet conditions fortotal internal reflection and reaches the interface between core andcladding is present. In addition, an adhesion between the cladding andthe core microscopically is not uniform in some portions, although itvisibly has very good transparency. The portions having nonuniformitiesbecome illuminating points.

The leaking light has generally a relatively low angle to a directionparallel to the cladding side face and largely contains components toemit out from the cladding side face. Accordingly, the optical fiberhaving a semi-transparent light-diffusive resin layer on the claddinghas sufficient uniformity of luminance over the longitudinal direction,but is poor in luminance strength because of the leakage of light, sothat the fiber is not used for a long light illuminant, like a neontube.

This necessitates that the light-diffusive layer be more closely adheredto the cladding surface to reduce the escapes of light having arelatively low angle to a direction parallel to the cladding side faceand to increase the escape of light having a relative high angle to thedirection parallel to the cladding side face.

In order to closely adhere a light-diffusive layer on the claddingsurface, some methods are already known. For example, there is a methodfor covering an electric wire with resin, which comprisescool-solidifying a resin melted mixture of a transparent polymer andwhite inorganic powder dispersed therein onto the surface of thecladding layer of the optical fiber. Another method comprises preparinga light-diffusive resin tube and inserting an optical fiber into thelight-diffusive resin tube.

However, the above-mentioned methods both include an additional step forcovering the light-diffusive layer on the cladding surface to result inincrease of cost for production.

In addition, since the light-diffusive layer is separately formed andcovered on the surface of the cladding, it is difficult to enhance anadhesion between the cladding and the light-diffusive layer. This oftencreates layer separation between the light-diffusive layer and thecladding because of a bending operation of the fiber, a change oftemperature and the like. Once the layer separation occurs, theluminance of the separated portion reduces and generates some differenceon luminance over the fiber. The fiber thus does not operate well as alight illuminant for illumination.

The present inventors have studied more about occurrence of layerseparation between the fiber and the light-diffusive layer and havefound that the occurrence of layer separation is observed more oftenwhen the optical fiber has a larger core diameter, especially 5 mm ormore core diameter. The reasons why the tendency exists will bedescribed below.

In case of glass fiber, the fiber is twisted to absorb bendingdeformation, to result in the glass fiber bending without breakage. Onthe other hand, if a glass article has a larger diameter than the fiber,for example a glass rod, it can not bend at all and therefore if toomuch bending force is applied on the rod it will break. It is generallytrue that a rod shape article having a large diameter does not twist atall against bending operation and does not absorb the bendingdeformation. Accordingly, the layer separation between the cladding andthe light-diffusive layer operates as the same as the glass rod andoccurs often when the optical fiber has a lager diameter.

The present invention will be explained referring with representativeembodiments. In the drawings attached to the present application, thesame numbers show same elements or equivalent elements. In FIG. 1, anoptical fiber 10 is indicated as one embodiment of the presentinvention. The optical fiber 10 has a core 1 at its center portion and acladding 2 surrounding the core 1.

The core is generally formed from polymer. The core formed from polymercan be obtained by polymerizing a polymerizable material. The core canaccept light without loss from a light source from one or both exposedends into the core. The core has a sufficient light transmittance andtransmits light from one end to the other end.

The core has a light transmittance of not less than 80%. By the term“light transmittance” herein is meant a value determined by aspectrophotometer using a light having a wavelength of 550 nm. Thepolymer for the core generally has a refractive index of 1.4 to 1.7.

The core preferably is a solid core formed from flexible polymer. Theflexible polymer can preferably be acrylic polymer, ethylene-vinylacetate copolymer, vinyl acetate-vinyl chloride copolymer or a mixturethereof. The polymer of the core can preferably be crosslinked in orderto enhance water resistance.

The polymerizable material for the core can be an acrylic monomermixture. The acrylic monomer mixture for the core contains (1) apolymerizable acrylic monomer not having a hydroxyl group in a moleculeand (2) a polymerizable hydroxyl-containing acrylic monomer. The term“acrylic monomer” used herein include either a monomer having an acrylicgroup or a monomer having a methacrylic group or both. Preferred is amethacrylate, i.e. methacrylic ester. The methacrylate can easilycontrol a core Tg to a suitable range and can effectively enhanceproperties in water resistance, light transmittance and the like. Thepolymerizable material for the core can also be a (meth)acrylic oligomerformed by reacting at least two monomers, as long as the technicaleffects of the present invention does not deteriorate. A crosslinkablemonomer having two or more functional groups can also be used inaddition to the mono-functional monomer.

Examples of the acrylic monomers not having an hydroxyl group are amethacrylate, such as methyl methacrylate, ethyl methacrylate, n-butylmethacrylate, 2-ethylhexyl methacrylate, isobutyl methacrylate, t-butylmethacrylate, lauryl methacrylate, dodecyl methacrylate and stearylmethacrylate. An acrylate not having hydroxyl group can be used inaddition to the methacrylate and include methyl acrylate, ethylacrylate, n-butyl acrylate, 2-ethylhexyl acrylate, isoamyl acrylate,lauryl acrylate, stearyl acrylate, isooctyl acrylate or the like. Anunsaturated acid, such as acrylic acid or methacrylic acid can also beused as the monomer.

Examples of the hydroxyl-containing acrylic monomers are 2-hydroxyethylmethacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl methacrylate,2-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 3-hydroxypropylacrylate, diethyleneglycol monomethacrylate, diethyleneglycolmonoacrylate, triethyleneglycol monomethacrylate, triethyleneglycolmonoacrylate and the like.

Examples of the crosslinking agents to crosslink the core polymer arepolyfunctional monomers, such as dially phtharate, triethyleneglycoldi(meth)acrylate, diethyleneglycol bisallylcarbonate and the like.

Preferred examples of the acrylic monomer mixtures for the presentinvention include:

(a) a mixture of 2-hydroxyethyl methacrylate, methyl methacrylate,n-butyl methacrylate and triethyleneglycol di(meth)acrylate;

(b) a mixture of 2-hydroxyethyl methacrylate, n-butyl methacrylate andtriethyleneglycol di(meth)acrylate; and

(c) a mixture of 2-hydroxyethyl methacrylate, n-butyl methacrylate,2-ethylehexyl methacrylate and triethyleneglycol di(meth)acrylate; andthe like.

In case where the core polymer is crosslinked by using a crosslinkingagent, an amount of the crosslinking agent can preferably be 0.01 to5.0% by weight, more preferably 0.1 to 4.5% by weight based on a totalweight of the polymerizable material. The core may also contain anadditive as long as the core does not deteriorate its properties.Examples of the additives are plasticizer, surfactant, colorant,stabilizer for heat, oxidation or ultraviolet light, and the like.

Any ingredient of the polymerizable material for the core can be variedso as to satisfy properties, such as softness, weather resistance,coloring resistance and water resistance. A length of the core maygenerally be 50 to 100 m, but is not limited thereto. For exhibiting thetechnical effects of the present invention, it is preferred that thecore has a length of 10 m or more, more preferably 15 m or more. Thecore generally has a cross section of about circle or ellipse in adirection of diameter, but does not limit thereto.

The core generally has a diameter of 3 to 30 mm. Diameters of less thanthe lower diameter generally do not fit an application to illumination,because an area of illuminating is too thin and small for observers tosee the illumination. On the other hand, diameters of more than largerlimitation would significantly have attenuation of luminance in alongitudinal direction and not enhance uniformity of luminance. Inaddition, the larger diameters reduce flexibility of the optical fiberand therefore do not form into an illumination apparatus containing thefiber having a desired shape. It is therefore preferred for showing goodperformance as an illuminant that the core has a diameter of 6 to 27 mm,more suitably of 7 to 20 mm.

The cladding 2, as explained above, integrally molds both the firstlayer 3 and the second layer 4 together. Preferably, the cladding 2 canbe formed by a co-extrusion method, in which two or more layers forforming the cladding are melt-extruded together to form layer and cooledto solidify. The method effectively enhances an adhesion between thelayers and does not increase number of steps for forming the cladding.The cladding of the present invention therefore can be produced as thesame as a conventional cladding having a single layer with the exceptionthat the cladding has plural layers.

Materials for forming each cladding layer are not limited, but generallyinclude a polymer, such as tetrafluoroetylene-hexafluoropropylenecopolymer (FEP), tetrafluoroethylene-hexafluoropropylene-vinylidenefluoride copolymer, trifluroethyle-vinylidene fluoride copolymer,polymethylpentene, ethylene-vinyl acetate copolymer, vinyl acetate-vinylchloride copolymer or the like. It is noted that the first layercontacting the core of the cladding has lower refractive index than thecore.

The cladding may contain some additive as long as the addition does notdeteriorate the performance of the present invention. Examples of theadditives are plasticizer, sufactant, curing agent, filler (e.g. whitepigment), colorant (e.g. dyestuff), stabilizer and the like.

The second layer being light diffusive property can generally be formedfrom a material containing fluorine-containing polymer and lightdiffusive particles dispersed in the fluorine-containing polymer. Anamount of the light diffusive particles is generally within the range of0.01 to 50% by weight, preferably 0.1 to 45% by weight, more preferably1 to 40% by weight based on a total weight of the second layer. Amountsof less than the lower limitation may not have sufficient luminance(e.g. 100 candera/m² or more for white illuminant) for an illuminantlike neon signs. Amounts of more than 50% by weight may not emit lighthaving enough luminance throughout a longitudinal direction.

The light diffusive particles can generally be glass beads or beadsobtained from another material, and inorganic particles, such astitanium dioxide or silicon dioxide. Concrete examples of the particlesare white inorganic particles having a refractive index of 1.5 to 3.0.Preferred examples of the white inorganic particles are barium sulfate(refractive index=1.5), magnesia (refractive index=1.8) and titania(titanium dioxide; refractive index=2.6.

The light diffusive particles can be other ones as long as they do notdeteriorate the technical effects of the present invention. In additionto the light diffusive particles, a colorant, such as fluorescent dye,can also be contained in the cladding layer to change white lightintroduced into the core to colored light and to emit it.

A transparency of the cladding first layer can be shown as lighttransmittance and is preferably more than 60%, more preferably more than70%, most preferably more than 90%. If the cladding first layer has toosmall light transmittance, the illuminant of the fiber would reduce.

The cladding first layer preferably has a thickness of 50 to 300 μm,more preferably 70 to 280 μm, most preferably 100 to 250 μm. It thecladding first layer has too small thickness, the attenuation ofluminance would be significant over in a longitudinal direction and doesnot enhance uniformity of luminance in the longitudinal direction. Incase where the core has a diameter of 5 mm or more, it is preferred thatthe cladding first layer should be as thick as possible for enhancingthe uniformity of luminance. If the cladding first layer is too thick, aluminance at portions away from the light-introduced point would reduceand the uniformity of luminance does not keep so far. In either case,the optical fiber is not suitable for an illuminant for illumination.

A thickness of the second layer of the cladding is not specificallylimited and can be selected such a range as not to make the claddingopaque. It is preferred that the second layer has a thickness of morethan 10 μm and the cladding totally has a thickness of not more than 2mm.

The optical fiber of the present invention is produced by preparing atube type cladding having a desired length and filling a polymerizablematerial into the tube type cladding, followed by polymerizing thematerial to form a polymerized core and a cladding covering the core.Detailed explanation of production is explained hereinafter.

First of all, a cladding (i.e. cladding tube) is prepared. The claddingis generally obtained by a co-extrusion method to form a cladding tubehaving desired thickness, diameter and length. The cladding producedabove is wound on a feed roll. The cladding wound on the feed roll iswound up on a wind-up roll. A combination of the feed roll and thewind-up roll is employed and a continuous cladding in a longitudinaldirection is sent from the feed roll to the wind-up roll, between whicha heating zone (a container containing a medium for heating, e.g. heatedwater container) is present and the cladding is driven through theheating zone.

The heating zone, that is, heating container may have two openingsthrough which the cladding is driven. The two openings are a claddinginlet opening in the side of the feed roll and a cladding outlet openingin the side of the wind-up roll. The heating container can be one havingone opening facing up side of a perpendicular direction. The cladding isintroduced inside the container through the one opening, its directionis changed near the bottom of the container and the cladding is thensent out through the same opening. As explained above, the cladding isdipped in a heating medium to finish polymerization of core and then anoptical fiber is taken out of the opening.

The polymerizable material for the core is generally filled in thecladding tube at a suitable pressure. In this method, the material isput into the cladding from the other end and then one end of thecladding is sealed. The sealing of the cladding can be conducted bycaulking the one end of the cladding with a metal cap or a valve.Filling-up of the polymerizable material into the cladding tube can beconducted by connecting one open end of the cladding with a tank for thepolymerizable material and the inside of the tank is pressured tocontinuously put the material into the cladding tube.

As mentioned above, the polymerizable material in the cladding is heatedin the heating zone to start and finish polymerization reaction toobtain the optical fiber having the core closely adhered to the claddinglayer.

Heating can be conducted at a temperature of 35 to 90° C., preferably 4085° C. A time for the cladding to stay in the heating zone is notspecifically limited, but generally is 10 minutes to 5 hours, preferably15 minutes to 3 hours. The cladding preferably has a length of 10 to3,000 m, preferably 20 to 2,000 m.

The cladding preferably has an elasticity of 10 to 700 MPa, preferably20 to 600 MPa. The “elasticity” of the cladding is a value at a heatingtemperature. The cladding preferably has a tube thickness of 0.01 to 2mm, preferably 0.05 to 1.5 mm, more preferably 0.2 to 1 mm. If it is toothin, the water resistance of the optical fiber would reduce. If it istoo thick, flexibility would lower. The inside diameter of the claddingcan be determined by the core diameter of the final optical fiber.

The optical fiber of the present invention is suitably used as a longilluminant of an illumination apparatus, equipped with an informationsign, such as an advertising board, a neon sign and a road sign.

The optical fiber of the present invention can emit light introducedfrom one end or both ends of the core to outside through side face orsurrounding face of the cladding. A light source can be a high-luminancelamp, such as a xenon lamp, a halogen lamp, a flush lamp. The lampsgenerally consume 10 to 500 W of electric power.

For example, the optical fiber of the present invention is used as along illuminant as shown in FIG. 5, thus forming an illuminationapparatus. In FIG. 5, a side light portion 30 formed by the long opticalfiber of the present invention shows a figure containing some curvedlines. In the illumination apparatus of the present invention, theilluminant containing such figures constitutes all or a portion ofadvertisement or guiding information.

A light transmitting portion 32 connecting a light source 31 with theside light portion 31 does not constitute the above-mentionedinformation. Accordingly, it is preferred that the light transmittingportion 32 is covered with light screening jacket (black soft vinylchloride resin) not to emit light.

The optical fiber of the present invention does not generate layerpeeling even by bending operation. Accordingly, the optical fiber isvery easy to form to a design containing the curved line as shown inFIG. 5, letters and symbols, which therefore shows satisfactoryperformance as an illuminant for illuminations.

In case where the optical fiber of the present invention is used as along illuminant for an illumination apparatus, the fiber preferably hasa length of 10 to 50 m, preferably 15 to 40 m if light is introducedfrom one end of the fiber by one light source, and has a length of 10 to100 m, preferably 15 to 80 m if light is introduced from both end by twolight sources.

EXAMPLES Example 1

A cladding having a first layer and a second layer, both beingintegrally formed, was prepared.

Two extruders were employed and an extruding end of each extruder wasconnected with a co-extrusion die. Into one of the extruder,tetrafluoroethylene-hexafluropropylene (FEP) resin (FEP 100J availablefrom Mitsui Du Pont Chemical Co., Ltd.) was put for a first layer. Intothe other extruder, a mixture of 100 parts by weight of FEP 100J and 1part by weight of NP 20 WH (FEP available from Daikin Industries Co.,Ltd.) was put for a second layer. The NP 20 WH resin contained 2.3% byweight of titanium dioxide. Accordingly, the second layer of thisexample had about 2.3% by weight of titanium dioxide (light-diffusiveparticles). The cladding first layer had a light transmittance of 9.5%.

By using the above mentioned extruders, a cladding for the Example wasprepared. The cladding had a tube shape and had openings at both ends.The first layer of the cladding had a transparent resin layer having athickness of about 200 μm and the second layer had a light-diffusivelayer having a thickness of about 450 μm. The cladding had an outsidediameter of about 15 mm.

As a polymerizable material for a core, a monomer mixture was preparedfrom 4 parts by weight of hydroxyethyl methacrylate, 80 parts by weightof n-butyl methacrylate, 16 parts by weight of 2-ethylhexyl methacrylateand one part by weight of triethyleneglycol dimethacrylate, into whichlauroyl peroxide (polymerization initiator) was added.

The polymerizable material was poured into the cladding from one end andthen the other end was sealed. The polymerization material was heated topolymerize in the heating zone by driving the sealed end of the claddingand continuously sending it, as contacting nitrogen gas from the otheropen end. The polymerized material formed a solid core to obtain a sidelight type optical fiber.

Example 2

A side light type optical fiber was prepared as generally described inExample 1, with the exception that an amount of NP 20 WH introduced intothe other extruder was changed from 1 part by weight to 20 parts byweight. The cladding second layer of the obtained fiber had about 38.3%by weight of light-diffusive particles (titanium dioxide).

Comparative Example 1

A side light type optical fiber was prepared as generally described inExample 1, with the exception that NP 20 WH was not used and only PEP100 J was employed.

Comparative Example 2

A side light type optical fiber was prepared as generally described inExample 1, with the exception that the resin for the extruder 1 waschanged to a mixture of FEP 100 J and NP 20 WH in an amount ratio of10:1 and the resin for the extruder 2 was only FEP 100 J. In thisexperiment, the first layer had light-diffusive properties and thesecond layer is transparent. The cladding second layer had a lighttransmittance of 95%.

(1) Determination of Side Light Luminance

A side light luminance was determined as following.

A metal halide lamp (LBM 130 H available from Sumitomo 3M Co.; consumedelectric power of 130 W) was connected with a core of an optical fiberat one end. The light source was put on and a luminance was determinedat a position apart from the light source in a given distance by aluminance meter available from Minolta Co., Ltd. as CS-100. Theluminance meter was positioned a point away from the side face of theoptical fiber in 60 cm. It is noted that an angle of a normal of an areareceiving light of the luminance meter with a longitudinal direction ofthe core was set 90°.

The results of the determination are shown in FIGS. 2 and 3.

FIG. 2 shows a change of luminance against a distance from a lightsource, that is an evaluation of uniformity of luminance over alongitudinal direction. The optical fibers of Examples 1 and 2 hadhigher uniformity compared with those of Comparative Example 2. InComparative Example 2, a luminance at a potion near the light source wasvery high, but the longer the distance of the measuring point from thelight source, the lower the luminance with relatively sharp steep. Onthe other hand, the optical fibers of Examples 1 and 2 had a very littledecline of luminance as parting a determining position from a lightsource.

In Comparative Example 1, luminance was very low throughout alongitudinal direction of the fiber, in comparison with the fibers ofExamples 1 and 2.

As is apparent from the above evaluation, the optical fibers of Examples1 and 2 are more suitable for a long illuminant for illumination thanthose of Comparative Examples 1 and 2.

FIG. 3 shows a change of luminance against a measuring angle at aposition of 2 mm from a light source. In FIG. 3, an axis of ordinatesindicates an angle of a normal of an area receiving light of a luminancemeter with a longitudinal direction of the core.

In this case, a direction which is parallel to the side surface of thecladding and faces to one end of connecting the light source is 180° anda direction which is parallel to the side surface of the cladding andfaces to the other end is zero degree, i.e. 0°.

The optical fiber of Example 1 enhanced a luminance of a light componentnear a perpendicular to the side area of the cladding, in comparisonwith the fiber of Comparative Example 1 which does not have a claddinglayer. Accordingly, the presence of the light diffusive layer at anoutermost surface can diffuse light having a low angle more toeffectively enhance a luminance of a light component near theperpendicular to the side area of the cladding.

Evaluation to Flexure

The optical fiber of Examples was cut into 1 m length and bent 10 timesat a bending angle of 90° with a curvature radius of 8 times of a corediameter (r=about 10 mm). After that, an evaluation was conducted aboutwhether layer separation occurred in the cladding or not. The opticalfibers of Examples 1 and 2 did not have the separation at all betweenthe first and second layers and also did not show any difference inappearance when connected with a light source between before and afterthe flexure test.

The side light type optical fiber of the present invention has uniformluminance of side light over a longitudinal direction, even if the fiberis relatively long. In addition, the optical fiber of the presentinvention can effectively prevent from layer separation between a firsttransparent layer contacting the core and a second light-diffusivelayer, even if it has a relatively larger core diameter.

1. A side light type optical fiber, comprising a core and a claddingdisposed around said core, said cladding is composed of a transparentfirst layer contacting said core, and a light diffusive second layerformed around the first layer, the both layers being integrally molded.2. The side light type optical fiber according to claim 1, wherein saidfirst layer has a thickness of 50 to 300 μm.
 3. The side light typeoptical fiber according to claim 1, wherein said core has a diameter of5 to 30 mm.
 4. The side light type optical fiber according to claim 1,wherein said cladding has a dual layer structure formed by aco-extrusion method of two materials for the first and second layers.